I have nothing to add that hasn't been said... and yea, the more you buy the more chances of something like this happening, though people who actually care about their eBay reputation care about helping you even if it wasn't what you were expecting.
I've only had one eBay problem that was recently. I bought a DVD and a week or so passed without any notification that the item was sent, so I contacted the seller. Looking back through his history, he'd canceled dozens of orders all of a sudden, and I think his reason was something to do with the place he was shipping them from.
At any rate, he quickly refunded my money without any discussion or delays, but it took me contacting him for him to get around to doing so.
It's also important to be sure and read the descriptions very carefully: